“Microfabrication” is the fabrication of extremely small structures, having features on the order of microns or nanometers. Microfabrication processes are used, for example, to produce integrated circuits, to build magnetic heads for reading and writing to magnetic media, and to fabricate miniature mechanical or electromechanical devices. During microfabrication, it is necessary to measure critical dimensions to ensure that the fabrication process is operating properly and that the parts being produced meet the product specification.
To determine critical dimension and other characteristics of microscopic objects, it is often necessary to obtain images of the object and then to measure the image. For example, microscopic structures are often viewed and measured using electron microscope images.
During the development of semiconductor devices, measuring the distribution of chemical elements present in the device structure is used to monitor process success and quality. One technique for identifying materials at points on a work piece is called energy dispersive x-ray spectroscopy (EDS or EDX), which can be used for elemental analysis or chemical characterization of a sample. In EDX, an electron beam is directed toward a sample and the energies of x-rays coming from the sample in response to the electron beam are measured and plotted as a histogram to form a spectrum. The measured spectrum can be compared to the known spectra of various elements to determine which elements and compounds are present.
EDX can be used to form elemental maps that show the spatial distribution of elements in a sample. The elemental map is created by moving an electron beam point by point over the sample. Several maps, each showing a single element, can be produced to show the composition of an area of a work piece. Other techniques, such as electron energy loss spectroscopy (EELS) and high-angle annular dark-field imaging (HAADF), can produce non-elemental images as well as elemental maps. EELS measures the energy absorbed as electrons pass through the sample. Different materials in the sample cause electrons to lose different amount of energy as they pass through, so the materials of the sample can be identified by measuring the amount of energy lost by the electrons. The energy of the electrons passing through the sample is measured and the energy loss is determined by subtracting the exiting energy from the energy of the electrons in the original beam. EELS can not only determine individual elements, but also their chemical states. HAADF is a method of mapping samples in a scanning transmission electron microscope (STEM). The images are formed by collecting only high angle scattered electrons using an annular dark-field detector and is sensitive to variations in the atomic number of atoms in a sample.
In order to make measurements on an image or perform other metrology activities, there has to be enough contrast to recognize features found in the image. In some methods, contrast for heavier elements, such as tungsten (W), tantalum (Ta), and hafnium (Hf), is different from the contrast for light elements, such as nitrogen (N), oxygen (O), and carbon (C). Different elements may appear in different images with different degrees of clarity. It can be difficult to determine measured geometric relationships between features composed of different elements, because the different elements may not both appear in the same image.